Calling on Dragons, though it does give us great background information on witches in general and Morwen in particular, is by far the weakest entry in the series.
My dislike, or perhaps I should say my lack of liking, for this book is primarily due to the character Killer, an enchanted bunny that Morwen finds in her garden. He's a hopping, talking MacGuffin; because a wizard was involved with his enchantment, Morwen takes him to the castle to consult with Mendanbar and Cimorene (who, incidentally, is pregnant), thus kickstarting the plot.
As the story goes on, far, FAR too much time is spent discussing Killer and letting us know what he said and did and how everyone else (especially Morwen's cats, each one of whom has to get in his/her say on everything*) reacted to it. And he's taken along on the main quest for a spurious reason. (The real reason is because the author thinks she needs to deliver exposition, and a character who doesn't know anything is a handy way to do that.)
Maybe if Killer were funny (which he's supposed to be), I could forgive this hamfisted approach, but he's just so annoying. Every time he talks, I want him to shut up because Cimorene, Kazul, or Morwen, all of whom are awesome, could be talking. And he keeps doing stupid things that end up being important later. He's like the Mary Sue of giant blue flying donkeys that are actually small brown rabbits.
Also, as mentioned previously, I read Searching for Dragons first and love it best, so the lack of Mendanbar in this book is a negative for me. It's still a nice, light read, though.
*YMMV if you are a cat person. I'm decidedly not, which may be part of why I found them annoying.